1. Field of the Invention
This invention relates to a system of controlling fluid flow, particularly in the presence of a large pressure drop, and more particularly in controlling the position of a choke with precision through the operation of a single hydraulic line input.
2. Description of the Prior Art
A choke valve has proven particularly wellsuited for determining the flow rate of fluids, even abrasive fluids, where the differential pressure drop across the valve is great, for example, on the order of 1000 pounds per square inch. Such a condition is encountered, for example, in the production of fluids of an offshore oil well where the valving of such fluids takes place typically at depths on the order of 2000 feet beneath the surface of the water.
U.S. Pat. No. 4,471,810 reveals a particularly worthy valving mechanism wherein the side ports around the inner cylinder of the choke valve are sized and positioned so that there is a linear change of flow rate with linear movement of the sleeve thereover, without causing great erosion in the flow path, as with the valve seat arrangement in U.S. Pat. No. 4,132,386.
The device shown in U.S. Pat. No. 4,471,810 is shown with a hand crank or other suitable valve closure mechanism where the parts are visible or otherwise readily manipulatible. For a subsea installation, however, the mechanisms shown for actuating the valve are unsuited.
It has heretofore been assumed that to obtain satisfactory control of the fluids encountered at subsea production environments it would be necessary to utilize mechanisms providing one or more of the following features: (1) use of proportional positioning of valve components by proportional hydraulic fluid application; (2) use of feedback in a servo-hydraulic system: (3) use of at least two hydraulic lines, one for operating the valve open and one for operating the valve closed; and (4) use of power for keeping the valve in a set position, even though the valve opening infrequently had to be modified.
Therefore, it is a feature of the present invention to provide an improved choke valve control operation utilizing only a single hydraulic line input.
It is another feature of the present invention to provide an improved choke valve control operation that does not consume power except when the valve opening is being changed.
It is still another feature of the present invention to provide an improved choke valve control operation that does not rely on the precision of the hydraulic control pressure to obtain precision of valve opening control.